1. Origin of the Invention
The invention described herein was made in the performance of work under a NASA Contract and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the contractor has elected not to retain title.
2. Field of the Invention
This invention relates to a new method and system for measuring relative humidity by using a known water-absorbent polymer operating as a capacitor circuit element in an RC oscillator circuit which exhibits a frequency that varies inversely with the partial pressure of the moisture vapor in an area to be measured. I have discovered that a water-absorbent polymer, when formed as a capacitor between porous electrically-conductive electrodes, will absorb moisture. The electrical properties of an RC oscillator, including the capacitor, vary linearly in frequency when compared to variations in relative humidity.
One preferred form of capacitor was subjected to a low DC voltage of about 3 to 3.2 volts. In this preferred embodiment the RC circuit element is formed from a Nafion capacitor which is operated in an RC oscillator circuit at the low DC voltage. At that low voltage the reactive current through the Nafion is sufficiently high to allow RC oscillation over a satisfactory humidity range. The frequency of oscillation varies in an essentially linear fashion with relative humidity which is represented by the moisture being absorbed into the Nafion. The range of oscillation is detected by a frequency detector. Thus, the amount of moisture varies the oscillator frequency and a simple, effective measuring system is attained.
3. Background Discussion
Nafion is water-absorbent and its use in a hygrometer as a moisture variable resistor is known in the art, as exemplified by U.S. Pat. No. 4,083,765. Various other moisture variable resistance structures using absorbent materials having a porous layer are shown in U.S. Pat. Nos. 4,143,177 and 4,203,087. A water-permeable layer in a capacitive hygrometer is disclosed in U.S. Pat. No. 4,482,581.
The inventor is aware of two prior art NASA new technology reports identified by NPO-13948 entitled "Long Lasting Solid Polymer Electrolytic Hygrometer" (Jet Propulsion Laboratory, May 1976) and NPO-15722 entitled "Trace Water Sensing in Space" (Jet Propulsion Laboratory, December 1981). These reports describe, as is well known, that typical polymer film material such as polyphosphoric acid film and sulphonated fluorocarbon polymers (DuPont Nafion) have excellent water absorption characteristics and superior chemical resistance. In these disclosures an attempt is made to read picoampere current variations as a measurement of local relative humidity. Such small currents result in an expensive and highly-sensitive system which is not only delicate but suffers from reliability, as discussed in more detail below.
The problem presented by the NASA prior art approaches is that, in both cases, the mode of water detection involves electrolysis of water absorbed on the polymer film and the presence of a chemical electrolyte. The use of high voltage in the electrolysis operation and/or the gases produced thereby tend to make the measurements unreliable. The electrodes become corroded and the corrosion also makes the measurements unreliable.
Although it has long been known as a desirable goal to monitor absorbed moisture on a polymer film by a simple, efficient nonelectrolytic system, attainment of that goal was not available until the advent of this invention.